name: deno-frontend description: Use when building web UIs with Fresh framework, using Preact components, or adding Tailwind CSS styling in Deno

Deno Frontend Development

Overview

This skill covers frontend development in Deno using Fresh (Deno's web framework), Preact (a lightweight React alternative), and Tailwind CSS. Fresh uses "island architecture" where pages render on the server and only interactive parts ship JavaScript to the browser.

When to Use This Skill

Creating a new Fresh web application
Building interactive UI components (islands)
Adding server-rendered pages and routes
Integrating Tailwind CSS for styling
Choosing between islands (client-side) vs components (server-only)
Working with Preact hooks and signals

Apply these practices when building web applications in Deno.

Fresh Framework

Reference: https://fresh.deno.dev/docs

Fresh is Deno's web framework. It uses island architecture - pages are rendered on the server, and only interactive parts ("islands") get JavaScript on the client.

Creating a Fresh Project

deno run -Ar jsr:@fresh/init
cd my-project
deno task dev    # Runs at http://127.0.0.1:5173/

Project Structure

my-project/
├── deno.json           # Config and dependencies
├── main.ts             # Entry point
├── fresh.gen.ts        # Auto-generated manifest (don't edit)
├── routes/             # Pages and API routes
│   ├── _app.tsx        # App layout wrapper
│   ├── index.tsx       # Home page (/)
│   └── api/            # API routes
├── islands/            # Interactive components (hydrated on client)
│   └── Counter.tsx
├── components/         # Server-only components (no JS shipped)
│   └── Button.tsx
└── static/             # Static assets

Key Concepts

Routes (routes/ folder)

File-based routing: routes/about.tsx → /about
Dynamic routes: routes/blog/[slug].tsx → /blog/my-post
API routes: routes/api/users.ts exports handlers

Layouts (_app.tsx)

import { PageProps } from "jsr:@fresh/core";

export default function App({ Component }: PageProps) {
  return (
    <html>
      <head>
        <title>My App</title>
      </head>
      <body>
        <Component />
      </body>
    </html>
  );
}

Async Server Components

export default async function Page() {
  const data = await fetchData(); // Runs on server only
  return <div>{data.title}</div>;
}

Islands (Interactive Components)

Islands are components that get hydrated (made interactive) on the client. Place them in the islands/ folder.

When to Use Islands

User interactions (clicks, form inputs)
Client-side state (counters, toggles)
Browser APIs (localStorage, geolocation)

Island Example

// islands/Counter.tsx
import { useSignal } from "@preact/signals";

export default function Counter() {
  const count = useSignal(0);

  return (
    <div>
      <p>Count: {count.value}</p>
      <button onClick={() => count.value++}>
        Increment
      </button>
    </div>
  );
}

Island Rules

Props must be serializable - No functions, only JSON-compatible data
Keep islands small - Less JavaScript shipped to client
Prefer server components - Only use islands when you need interactivity

Preact

Preact is a 3KB alternative to React. Fresh uses Preact instead of React.

Preact vs React Differences

Preact	React
`class` works	`className` required
`@preact/signals`	`useState`
3KB bundle	~40KB bundle

Hooks (Same as React)

import { useState, useEffect, useRef } from "preact/hooks";

function MyComponent() {
  const [value, setValue] = useState(0);
  const inputRef = useRef<HTMLInputElement>(null);

  useEffect(() => {
    console.log("Component mounted");
  }, []);

  return <input ref={inputRef} value={value} />;
}

Signals (Preact's Reactive State)

Signals are Preact's more efficient alternative to useState:

import { signal, computed } from "@preact/signals";

const count = signal(0);
const doubled = computed(() => count.value * 2);

function Counter() {
  return (
    <div>
      <p>Count: {count}</p>
      <p>Doubled: {doubled}</p>
      <button onClick={() => count.value++}>+1</button>
    </div>
  );
}

Benefits of signals:

More granular updates (only re-renders what changed)
Can be defined outside components
Cleaner code for shared state

Tailwind CSS in Fresh

Fresh 2.0 uses Vite for builds, which means Tailwind integrates via the Vite plugin.

Setup

In deno.json:

{
  "imports": {
    "@tailwindcss/vite": "npm:@tailwindcss/vite"
  }
}

In vite.config.ts:

import { defineConfig } from "vite";
import tailwindcss from "@tailwindcss/vite";

export default defineConfig({
  plugins: [tailwindcss()],
});

Usage

export default function Button({ children }) {
  return (
    <button class="px-4 py-2 bg-blue-500 text-white rounded hover:bg-blue-600">
      {children}
    </button>
  );
}

Best Practices

Prefer utility classes over @apply
Use class not className (Preact supports both, but class is simpler)
Dark mode: Use class strategy in tailwind.config.js

<div class="bg-white dark:bg-gray-900">
  <p class="text-gray-900 dark:text-white">Hello</p>
</div>

Building and Deploying

Development

deno task dev      # Start dev server with hot reload (http://127.0.0.1:5173/)

Production Build

deno task build    # Build for production
deno task preview  # Preview production build locally

Deploy to Deno Deploy

deno task build           # Build first
deno deploy --prod        # Deploy to production

Quick Reference

Task	Command/Pattern
Create Fresh project	`deno run -Ar jsr:@fresh/init`
Start dev server	`deno task dev` (port 5173)
Build for production	`deno task build`
Add a page	Create `routes/pagename.tsx`
Add an API route	Create `routes/api/endpoint.ts`
Add interactive component	Create `islands/ComponentName.tsx`
Add static component	Create `components/ComponentName.tsx`

Common Mistakes

Putting too much JavaScript in islands

// ❌ Wrong - entire page as an island (ships all JS to client)
// islands/HomePage.tsx
export default function HomePage() {
  return (
    <div>
      <Header />
      <MainContent />  {/* Most of this doesn't need interactivity */}
      <Footer />
    </div>
  );
}

// ✅ Correct - only interactive parts are islands
// routes/index.tsx (server component)
import Counter from "../islands/Counter.tsx";

export default function HomePage() {
  return (
    <div>
      <Header />
      <MainContent />
      <Counter />  {/* Only this needs to be an island */}
      <Footer />
    </div>
  );
}

Passing non-serializable props to islands

// ❌ Wrong - functions can't be serialized
<Counter onUpdate={(val) => console.log(val)} />

// ✅ Correct - only pass JSON-serializable data
<Counter initialValue={5} label="Click count" />

Using className instead of class

// ❌ Works but unnecessary in Preact
<div className="container">

// ✅ Preact supports native HTML attribute
<div class="container">

Forgetting to build before deploying Fresh 2.0

# ❌ Wrong - Fresh 2.0 requires a build step
deno deploy --prod

# ✅ Correct - build first, then deploy
deno task build
deno deploy --prod

Creating islands for non-interactive content

// ❌ Wrong - this doesn't need to be an island (no interactivity)
// islands/StaticCard.tsx
export default function StaticCard({ title, body }) {
  return <div class="card"><h2>{title}</h2><p>{body}</p></div>;
}

// ✅ Correct - use a regular component (no JS shipped)
// components/StaticCard.tsx
export default function StaticCard({ title, body }) {
  return <div class="card"><h2>{title}</h2><p>{body}</p></div>;
}

deno-frontend

SKILL.md